The peak luminosity determines the number of spectral states which are
seen during the outburst. Systems with peak luminosity below \(\sim 0.1L_{Edd}\)  remain in the low/hard state (LHS), where the energy output
peaks at \(\sim \)  100 keV. Brighter systems instead show a distinct
transition to a much softer thermal dominated state (TDS, also
termed a high/soft state) which peaks at \(\sim \)  1 keV, while
the brightest systems can also show a very high state (VHS,
alternatively steep power law state). These dramatic changes in the
spectrum are correlated with equally dramatic changes in the rapid
variability properties (see e.g. [1]}) and the jet
[2]}, implying distinct changes in the nature and geometry of
the accretion flow. These can be plausibly explained if there is a
hot, optically thin, geometrically thick solution to the accretion
flow equations at low luminosities ([3]};
[4]}).This can be put together with a cool, optically thick,
geometrically thin disc [5]} in the truncated disc/hot flow
model where the disc progressively replaces more of the inner hot flow
as the mass accretion rate increases. The distinct hard/soft spectral
transition then marks the point at which the cool disc extends down to
the last stable orbit, replacing all the hot flow ([6]};
DGK07).
